1. Field of the Invention
The present invention relates generally to noble metal nanostructures, and more particularly, to noble metal nanostructures having fewer surface defect sites, improved dispersibility, decreased diameter and improved electrocatalytic properties.
2. Description of the Related Art
Nanomaterials provide size-dependent optical, electronic, magnetic, thermal, mechanical, chemical, and physical properties, which are distinctive from their bulk counterparts as well as from the atomic or molecular precursors from which the nanomaterials were derived. (See Xia et al., Adv. Mater. 2003, 15, (5), 353-389; Mao et al., Small 2007, 3, (7), 1122-1139). A nanostructure is a material particle with one or more physical dimensions that are less than or equal to 100 nm. A nanostructure includes but is not limited to nanowires, nanotubes, nanoparticles, nanorods, nanospheres, nanofilm, nanocages, nanofibers, nanoflakes, nanoflowers, nanofoam, nanopillars, nanoplatelets, nanorings, nanoshells, nanoneedles, nanodendrites, nano-sea-urchins, nanopyramids, nanotriangles, and nanocubes.
A metallic nanostructure is a nanostructure with a chemical composition including any element or mixture of transition metals, including elemental, doped nanostructures, alloyed/solid-solution, and intermetallic nanostructures, as described below. A transition metal is a metal that falls within a set of metallic elements occupying a central d-block (Groups IVB-VIII, IB, and IIB, or 4-12) in the periodic table. Chemically, transition metals show variable valence and a strong tendency to form coordination compounds, and many of their compounds are colored. The noble metals are a subset of the transition metals that are resistant to corrosion and oxidation in moist air, unlike the other transition metals. Noble metals tend to be precious, often due to their rarity in the Earth's crust. The noble metals include platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), osmium (Os), iridium (Ir), silver (Ag), and gold (Au). Examples of transition metal nanostructures are described by U.S. Pat. No. 7,481,990 and U.S. Pat. No. 7,147,834, the contents of which are incorporated herein by reference.
Generally, there are four classifications of metallic nanostructures; an elemental metallic nanostructure, a doped metal nanostructure, an alloyed metal nanostructure, and an intermetallic nanostructure. An elemental metallic nanostructure is composed of a single transition metal and no other elements. Further, noble metals can be combined with other transition metals giving rise to the three additional classifications of noble metal nanostructures.
More specifically, a doped metal nanostructure is composed of a uniform solid mixture in which one primary transition metal element is combined with a quantity of another transition metal not exceeding 50% of the total composition, for the purpose of introducing new physical properties to the mixture or to enhance one or more of the physical properties of the primary transition metal. An alloyed metal nanostructure, also known as a Solid Solution Metal Nanostructure, is composed of a uniform solid mixture in which two or more transition metal elements can be combined into a single discrete nanostructure at any level of composition. An intermetallic nanostructure is composed of a mixture of two or more transition metal elements having a defined solid crystalline phase of the mixture different from that of the two transition metal elements.
A nanowire is a one-dimensional (1-D) nanostructure possessing two physical dimensions that are less than or equal to 100 nm with the third dimension being unconstrained with an aspect ratio defined as a ratio between the unconstrained dimension and the constrained dimensions, thereby providing a property of anisotropy. The concept of anisotropy is described further by U.S. Pat. No. 7,585,474, the content of which is incorporated herein by reference.
A growing demand for efficient, low-cost renewable energy has sparked great scientific interest in the development of nanomaterials for use as electrocatalysts in Oxygen Reduction Reactions (ORR). State-of-the-art ORR electrocatalysts include platinum zero-dimensional (0-D) nanoparticles supported on mesoporous carbon supports. However, there are several problems associated with 0-D, i.e., spherical or symmetrical, nanoparticle morphologies.
0-D nanoparticle morphologies maintain a proportionally larger number of lattice boundaries, defect sites and Low Coordination Atoms (LCAs) on their surfaces as compared with associated 1-D-type analogues. Defect sites and LCAs are less catalytically active than smooth crystal planes because of local differences in coordination geometry and surface energy, which can change, for example, the interface between exposed platinum atoms and the oxygen adsorbate. As a result, kinetics of these nanoparticulate electrocatalysts are slow due to an observed overpotential and catalytic inhibition due to adsorption of surface O-H groups at potentials below 1 V.
Moreover, 0-D electrocatalysts lack durability for long-term applications in fuel cells due to irreversible oxidation of surface atoms that occurs in defect sites. These factors contribute to high precious metal loadings of 0.4-0.8 mg/cm, rendering impractical use in commercialized fuel cells.
By contrast, single-crystalline, anisotropic 1-D structures possess: (a) high aspect ratios, (b) fewer lattice boundaries, (c) long segments of smooth crystal planes, and (d) a low number of surface defect sites and LCAs, all of which are desirable attributes for fuel cell catalysts. Moreover, the 1-D geometry allows for preferential exposure of low-energy crystal facets that are highly active in ORR. This factor contributes to a delay in surface oxidation at higher potentials, thereby enhancing ORR kinetics. In addition, 1-D structures maintain improved electron transport characteristics due to path directing effects of the structural anisotropy.
Further, 1-D morphologies lead to improved performance when platinum nanostructures are used as fuel cell electrocatalysts. Specifically, 1-D platinum nanostructures have fewer defect sites and have a higher number of surface atoms with the potential for higher degrees of coordination. In particular, one study found that small diameter platinum nanowires grown directly on an amorphous carbon support showed an area specific activity for the ORR of 275 μA/cm2, which is more than three times that of a commercial nanoparticle cathode. (See Sun et al., Adv. Mater., 2008, 20, 3900-3904).
In addition, unsupported platinum nanotubes prepared by Yan et al. display a four-fold enhancement in area-specific activity compared with carbon-supported platinum nanoparticles. (See Yan et al, Agnew. Chem., Int. Ed., 2007. 46, 4060-4063). Moreover, Wong et al. demonstrate that unsupported, polycrystalline platinum nanotubes synthesized by a facile template directed method can more than double the area-specific ORR activity as compared with commercial carbon-supported platinum nanoparticles. (See Wong et al. J. Phys. Chem. C, 2009, 113, 5460).
Based on these advantages, nanostructures have attracted extensive synthetic attention as a result of their size-dependent properties. (See, for example, J. Hu, et al., Acc. Chem. Res., 1999, 32, 435; G. R. Patzke, et al., Angew. Chem., Int. Ed., 2002, 41, 2446; Y. Xia, et al., Adv. Mater., 2003, 15, 353; and C. N. Rao, Dalton Trans., 2003, 1). Part of the challenge of developing practical nanoscale devices and catalysts for a variety of applications is the ability to synthesize and characterize these nanostructures to rationally exploit their nanoscale optical, electronic, thermal, and mechanical properties. Ideally, the net result of nanoscale synthesis is the production of structures that achieve monodispersity, stability, and crystallinity with a predictable morphology. Many of the synthetic methods used to attain these goals have been based on principles derived from semiconductor technology, solid state chemistry, and molecular inorganic cluster chemistry.
However, a need still exists for a scalable method to synthesize ultra-thin, highly active noble metal catalysts with few surface defects and LCAs, particularly to minimize precious metal loading.